The present invention relates to methods of forming propped fractures in portions of subterranean formations. Producing wells (such as hydrocarbon producing wells and water producing wells) are often stimulated by hydraulic fracturing treatments. In hydraulic fracturing treatments, a viscous fracturing fluid, which also functions as a carrier fluid, is pumped into a portion of a subterranean formation at a rate and pressure such that the subterranean formation breaks down and one or more fractures are formed. Typically, particulate solids, such as graded sand, are suspended in a portion of the fracturing fluid are then deposited in the fractures. These particulate solids, or “proppant particulates,” serve to prevent the fractures from fully closing once the hydraulic pressure is released. By keeping the fracture from fully closing, the proppant particulates aid in forming conductive paths through which fluids may flow.
Commonly used proppant particulates generally comprise substantially spherical particles, such as graded sand, bauxite, ceramics, or even nut hulls. Generally, the proppant particulates are placed in the fracture in a concentration such that they formed a tight pack of particulates. Unfortunately, in such traditional operations, when fractures close upon the proppant particulates the particulates may crush or become compacted, potentially forming non-permeable or low-permeability masses within the fracture rather than desirable high-permeability masses; such low-permeability masses may choke the flow path of the fluids within the formation. Furthermore, the proppant particulates may become embedded in particularly soft formations, negatively impacting production.
The degree of success of a fracturing operation depends, at least in part, upon fracture porosity and conductivity once the fracturing operation is stopped and production is begun. Traditional fracturing operations place a large volume of proppant particulates into a fracture and the porosity of the resultant proppant packed fracture is then related to the interconnected interstitial spaces between the abutting proppant particulates. Thus, the resultant fracture porosity from a traditional fracturing operation is closely related to the strength of the placed proppant particulates (if the placed particulates crush then the pieces of broken proppant may plug the interstitial spaces) and the size and shape of the placed particulate (larger, more spherical proppant particulates generally yield increased interstitial spaces between the particulates).
One way proposed to combat problems inherent in tight proppant particulate packs involves placing a much reduced volume of proppant particulates in a fracture to create a high porosity pack of proppant particulates, such a pack may be referred to as a partial monolayer. In partial monolayers, the proppant particulates within the fracture may be widely spaced, but they are still sufficient to hold the fracture open and allow for production. Partial monolayers may allow for increased fracture conductivity due, at least in part, to the fact the produced fluids may flow around widely spaced proppant particulates rather than just through the relatively small interstitial spaces in a traditional packed proppant particulate bed.
While the concept of partial monolayers for use in fracturing has been investigated in the industry, the concept has not been successfully applied for a number of reasons. One problem is that successful placement of a partial monolayer of proppant particulates presents unique challenges in the relative densities of the particulates versus the carrier fluid. Another problem lies in the fact that placing a proppant that tends to crush or embed under pressure may allow the fracture to pinch or close in places once the fracturing pressure is released. Yet another problem is that the particulates may tend to space unevenly or space individually rather than in aggregates, and thus they create thinner or smaller than desired fracture once the fracturing pressure is released.